Adding Value to AD

The U.K.’s National Centre for Biorenewable Energy, Fuels and Materials has recently released a report that finds using biomass crops can play a valuable role in on-farm anaerobic digestion (AD).

The main point of the report, which was released in May, is that anaerobic digesters operate more efficiently when biomass crops are used along with slurry and manure. “Biomass that has already been digested once, for example in a cow, will have a lower yield than biomass that hasn’t been digested before,” explains Matthew Aylott of the NNFCC. “In general, adding crops to an anaerobic digester will increase biogas yields because there are more sugars and proteins in the crop biomass for microorganisms to consume than in slurry or manure.”

Additionally, the report finds that AD can offer farmers a guaranteed market for their surplus crops or residues, a chance to use different cropping rotations and the opportunity to diversify their income. Crops examined in the report are those considered most appropriate for production in the U.K. as supplementary feedstock for AD, Aylott says. These include corn and grass silage and whole crop cereals such as wheat. Farms vary from small- to average-sized dairy farms (130 cows with followers), medium-scale (250 cows with followers) and large-scale (large, mixed farm or collaborative venture assuming 500 dairy cows with followers housed year-round).

The most attractive option for each farm varies according to farm size, according to Aylott. “At small scale, the option generating the most favorable internal rate of return under the U.K. electricity feed-in tariff system, which offers AD plants a fixed income for every kilowatt hour of electricity generated, is the slurry-only plant, primarily due to the cost of feedstock impacting the overall profitability.”

However, if the aim is to increase energy output to warrant heat and power generation, then some crop material should be added, he says. Ideally that would be grass or maize silage, as slurry alone has such a low energy value.

At medium scale, the most attractive economic option is again the slurry-only model, followed closely by the 70:30 slurry-to-crop ratio. “This example shows that inclusion of a relatively small volume of crop material can increase energy output tenfold for only three times the capital cost,” Aylott says. “Although the cost of production and the energy yield remains constant, the capital and operational costs do not increase proportionate to the scale of the plant.”

The most attractive option for large-scale farms is a higher crop-to-slurry ratio of 60:40, while remaining under the 500-kilowatt threshold for the higher feed-in tariff payment. This maximizes economic returns and energy output, Aylott says. He notes that when the report was composed, the U.K.’s Renewable Heat Incentive details were not available and therefore have not been taken into consideration.

While the benefits can be numerous, using crops in AD processes doesn’t come without challenges, the report recognizes. “A high proportion of crop material, typically above 50 percent inclusion, would require different AD technology due to the high dry matter content of the feedstock mix, or for water to be added or recirculated through the system,” Aylott says. “This increases the size of the digester tank(s) required, and as a result increases investment costs. Although this is technically possible and such plants are already developing, it is clearly not the intention of the feed-in tariffs or RHI to encourage wide-scale deployment of crop-based plants, which are more common in countries like Germany. As a consequence, the tariffs are not currently optimized for energy production.”

The research is now being fed back to the U.K. Department of Energy and Climate Change to support policy development on AD, including the current review of the feed-in tariffs support scheme.